1. Field of the Invention
The present invention is related to a radiation detector which is suited to application to radiation imaging apparatuses, such as X-ray imaging apparatuses.
2. Description of the Related Art
Currently, various X-ray imaging apparatuses that employ solid state radiation detectors (having semiconductors as the main portions thereof) as X-ray image information recording means have been proposed and are in practical use in the field of X-ray (radiation) imaging for the purposes of medical diagnosis and the like.
Various types of radiation detectors to be employed by the X-ray imaging apparatuses have been proposed. From the viewpoint of a charge generating process for converting X-rays to electric charges, there are radiation detectors of the optical conversion type (indirect conversion type), and radiation detectors of the direct conversion type. In a radiation detector of the indirect conversion type, fluorescence generated from phosphors due to the irradiation of radiation is detected by a photoconductive layer. Signal charges obtained by the photoconductive layer are temporarily accumulated. Then, the accumulated charges are converted to image signals (electric signals) and output. In a radiation detector of the direct conversion type, electric charges are generated within a photoconductive layer due to irradiation of X-rays. These signal charges are collected by charge collecting electrodes, and temporarily accumulated in a charge accumulating section. The accumulated charges are converted into electric signals and then output.
There are two main types of methods for reading out accumulated electric charges as well. One is an optical readout method that irradiates detectors with readout light (readout electromagnetic waves) to perform readout. The other is a TFT readout method that scans and drives TFT'S (Thin Film Transistors) which are connected to charge accumulating sections, to read out the accumulated charges.
The radiation detectors described above generate electric charges corresponding to X-ray energy when X-rays are irradiated onto charge generating layers provided in the detectors. The generated electric charges are read out as electric signals, and the aforementioned photoconductive layer functions as a charge generating layer. Conventionally, materials such as amorphous selenium (a-Se), PbO, PbI2, HgI2, BiI3, and Cd(Zn)Te have been used to form photoconductive layers.
Forming a-Se layers having large areas is possible by utilizing thin film forming techniques such as vacuum vapor deposition. However, there is a trend for many structural faults to be included therein due to the amorphous nature of a-Se, which leads to deterioration of sensitivity. It is common to dope a-Se with an appropriate amount of impurities, to improve the performance characteristics thereof. For example, U.S. Patent Application Publication No. 20030223534 discloses a recording photoconductive layer formed by a-Se doped with an alkali metal within a range from 0.01 ppm to 10 ppm. As another example, U.S. Pat. No. 3,685,989 discloses a recording photoconductive layer formed by a-Se doped with Na at 100 ppm.
The positive hole transport properties of a-Se were sufficient, because it is a P type semiconductor. Therefore, it had been considered that improving the electron transport properties of a-Se was necessary to obtain high sensitivity. U.S. Patent Application Publication No. 20030223534 and U.S. Pat. No. 3,685,989 both disclose a-Se having improved electron transport properties, by being doped with Na. The improved electron transport properties were obtained as a result of reduction of electron capturing centers from among the charge faults of a-Se.
However, in order to further improve sensitivity, it is necessary to improve not only the electron transport properties but the positive hole transport properties as well, in order to transport generated charges to the electrodes at both sides of the photoconductive layer. As a result of investigation by the present inventors, it was discovered that positive hole transport properties could be improved while maintaining high electron transport properties, by doping a-Se with alkali metals at a predetermined low concentration.